It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed. it has been found desirable to add a charge director compound to the liquid toner comprising the thermoplastic resin, dispersant nonpolar liquid and generally a colorant. Such liquid developers provide images of good resolution, but it has been found that the charging and image quaIity are particularly pigment dependent. For example, liquid electrostatic developers wherein copolymers of ethylene and carboxylic acid containing monomers are used to form the resin particles proVide good image quality particularly when carbon black pigments are present in the formulation. Removal of the pigment, however, normally results in poor charging and resultant poor images. Charging and image quality of such developers can be improved independent of pigment by the use of strongly acidic resins. In Larson and Trout, U.S. Ser. No. 880,155, filed June 30, 1986, resin particles in a liquid electrostatic developer are disclosed wherein a constituent containing at least one acidic constituent having a pKa of less than 4.5 measured at 25.degree. C. in water can be present as part of a homopolymer or copolymer. The preparation of such resins require skilled chemists and the procedures are time consuming and expensive. In addition, some of these resins may require special care in processing liquid electrostatic deVelopers prepared using them.
It has been found that the above disadvantages can be overcome and liquid electrostatic developers prepared containing a resin blend ionic or zwitterionic compound soluble in nonpolar liquid which have excellent negative charging characteristics, good image quality, with excellent resolution, evenness of toning, solid area coverage, and toning of fine details independent of pigment.